Electricity and water don't usually mix well (toaster + bathtub = problem), but this is just a concept anyway, and concept cars are allowed to take liberties with little things like “physics” and “reality” now and again. That's especially the case when the concept looks as drop-dead-sexy as the Manta.
The Manta amphibious vehicle concept is – like the Ventile posted earlier this week – part of the 2011 Michelin Design Challenge, and seems to use a combination of hull-mimicking bodywork and pivoting, hub-centric electric motors powering two propeller-shaped wheels on individual “steering axes” (my term, since there is no visible rudder) to get around on land and sea.
Is it viable? Probably not, but GM has made significant strides in improving battery safety underwater, and new technologies for improving battery range and durability are popping up all the time … so, who knows? Something like this Manta concept might just be the personal watercraft of the (green energy) future!